sounds like it could be a loose spark plug. Get in there and torque them down. If you have a torque wrench the spec is
Spark Plug to Cylinder Head . . . . . 30 Nm (22 ft-lb) (Bentley, page 020-11)
If you need to replace them they are Bosche F7 L DCRs (Bentley, page 020-11)

Also make sure the coil boots are in good shape and not damaged

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1995 525iA 250k mi Shifted Performance E34
2006 325i/6 48k mi
2003 350Z 87k mi Shifted Performance 350Z


Garden State Euros